Mitochondrial dysfunctions are commonly observed in cardiac injury and heart failure. Despite significant progress in past decades, the molecular sequence of events that transition the cardiac mitochondrial proteome from a healthy to dysfunctional state is not known. Current technologies that measure static protein abundance are insufficient to fully understand the time-dimensional features of this process, necessitating new conceptual and technical approaches to promote a more comprehensive understanding and to drive discovery of therapies. In this proposal, we will use alterations in mitochondrial protein turnover to mechanistically define cardiac remodeling, and we will investigate the differential role of intramitochondrial and extramitochondrial protein degradation pathways in transitioning the mitochondrial proteome from a healthy to diseased state. We hypothesize that ROS-induced damage to the mitochondrial proteome triggers alterations in protein half-lives-the functional window-of proteins comprising key functional components of mitochondria (e.g., ETC), which induces mitochondrial malfunction and propels cardiac remodeling. We postulate that protein turnover rates will unveil time-dependent molecular features of pathology that are otherwise masked in steady-state protein measurements, thus providing novel mechanistic insights into cardiac hypertrophy. The rationale for these studies is supported by recent discoveries on the importance of protein quality control and mitochondrial dynamics in cardiac phenotypes, and by our preliminary investigations regarding the effects of ROS on intra-mitochondrial protease activities. The proposed experiments will examine (i): protein turnover during cardiac mitochondrial remodeling, and (ii) the manner in which protein degradation pathways affect cardiac phenotypes. Aim 1 capitalizes on a novel heavy water (D2O) labeling strategy recently developed in our lab, which examines the temporal dynamics of the mitochondrial proteome in three settings: (i) isoproterenol (ISO)-induced remodeling; (ii) injury by elevated ROS; and (iii) cardioprotection by active PKC?. We anticipate that our findings will identify the protein turnover perturbations during remodeling and will elucidate their functional consequences. Aim 2 will determine proteolytic activities of individual protein degradation pathways targeting cardiac mitochondrial proteins. We expect these studies to determine how each proteolytic pathway changes during ISO treatment, ROS elevation, and PKC? cardioprotection to regulate mitochondrial dynamics. Finally, Aim 3 will integrate the knowledge gained from Aim 1 and Aim 2 to create and refine a translational model for measuring protein turnover in clinical studies. We expect these pilot studies to lay the foundation for comprehensive investigations of protein quality control in human heart diseases. Together, we expect these aims to reveal new insights on the nature of mitochondrial dynamics at the individual protein level in the heart.